Ultraviolet germicidal apparatus and method

ABSTRACT

A germicidal method and apparatus for destroying airborne pathogenic bacteria such as tuberculosis bacteria using ultraviolet light. Air is drawn through a filter and into a sterilization chamber that is irradiated with ultraviolet light, and out through an exhaust opening. Consideration for the characteristics of the room in which the apparatus is installed and the positioning of the installation allows effective prevention of transmission of disease through expectoration and inhalation of airborne microdroplets of bacteria-containing sputum. The filter is of the low-density type which traps large particulates, but not small particulates of the size of the microdroplets, so that the filter does not become a bacteria colonization site. Baffles on the air intake opening and air exhaust opening to prevent ultraviolet light from escaping into the environment. The sterilization chamber is constructed such that the air passes the ultraviolet light bulbs twice as it circulates therethrough.

This is a continuation of application Ser. No. 08/087,178 filed Jul. 2,1993, now abandoned which is a continuation-in-part of application Ser.No. 07/960,085 filed Oct. 9, 1992 now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of germicidal systemsemploying bacteria-destroying ultraviolet lights. In particular, thepresent invention relates to a system for producing an air flow througha baffled ultraviolet sterilization chamber mounted behind a wall orceiling, wherein the ultraviolet light intensity, the air residencytime, and the air exchange rate for the air volume in a given space, aresuch that a percentage of tuberculosis bacteria are destroyed thateffectively prevents transmission of such disease by airborne sputum.

BACKGROUND OF THE INVENTION

Tuberculosis is the most common cause of death from infectious diseasein the world today. It infects millions of people each year and causeshundreds of thousands of fatalities. The disease is particularlyprevalent in less-industrialized countries where high populationdensities, poor sanitary conditions and a high percentage of individualsin poor health contribute to the spread of infectious diseases.

After a long period of declining rates of tuberculosis infection in theUnited Sates, it is believed that the infection rate is now increasing.The increasing rate is apparently due to a combination of factors. Onefactor is undoubtedly increased immigration from parts of the world withhigh rates of infection. For example, in the United States the case rateof tuberculosis per 100,000 population was 9.3 in 1985, resulting inover 22,000 cases and over 1,200 deaths. In Southeast Asia, both thecase rate and the death rate are believed to be many times that, andimmigrants from that part of the world now constitute 3 to 5% of newcases in the United States.

Another factor related to increased rates of tuberculosis infectionappears to be the use of living quarters with high population densitiesand less-than-ideal sanitary conditions for persons in ill health whoare susceptible to the disease. Such conditions are commonly found inshelters for the homeless, prisons and some nursing homes. Anotherimportant factor in the increased rate of infection is infections amongpatients with Acquired Immune Deficiency Syndrome (AIDS) and intravenousdrug users.

Another reason for the recent increased incidence of tuberculosis isprobably the failure of many medical professionals to diagnose and treatthe disease early and properly. The relative rareness of the disease inthe United States since the early epidemics resulted in an entiregeneration of health care workers without much experience in thedisease. Further, diagnosing the disease is not always easy, for thesymptoms are similar to the symptoms of many other disorders. Therefore,the disease is often misdiagnosed and mistreated, and the degree ofinfectiousness of the disease is underappreciated.

Even after it is recognized that a set of symptoms may indicatetuberculosis, the tests for the disease are somewhat imprecise and tendto require judgment by an experienced professional. For example, onediagnostic tool is chest x-rays which typically show apical-posteriorsegment cavitary changes in tuberculosis infected patients. However, inelderly individuals—who comprise a relatively large proportion oftuberculosis patients—lobar or patchy lower-zone shadows may simulatebacterial or aspiration pneumonia. Also, x-rays in the elderly maymislead the physician by showing a solitary pulmonary nodule or apleural effusion. Another important tuberculosis test is thetuberculosis skin test, but a major disadvantage to the tuberculosisskin test is that it generates a high number of both false-positive andfalse-negative results. The most precise test is microscopic examinationof a sputum sample, but this test may require the use of at least threeseparate samples of sufficient volume, which may require gastricaspiration or bronchoscopy in patients with low sputum production.

The normal body reaction to infection by tuberculosis bacteria is tobuild a fibrous wall around each bacterium. Initially, a person may beunaware of any infection, but over a period of months or even years theinfection produces inflammation and eventually destruction of tissue.The manifestations as the disease progresses generally include cough,fever, night sweats, hemoptysis, chest pain, weight loss and malaise.The usual treatment for tuberculosis is administration of drugs over aperiod of many months such as isoniazid, rifampin and pyrazinamide andethambutol. Persons recently infected but with no active disease areusually given isoniazid preventive therapy, particularly if they haveother risks such as malnutrition, gastrectomy, diabetes mellitus,pneumoconiosis, malignancy or if for some reason they haveimmunosuppression such as from corticosteroid therapy, renal impairmentor HIV infection. In short, tuberculosis in a normal healthy patient istypically a disease that is curable by drugs, although the drug therapyis quite prolonged. A serious concern—and yet another reason for therecent increase in tuberculosis—is the development of drug-resistanttuberculosis. It is estimated that at least 5% of new cases areresistant to the usual drug therapy, and that the percentage in someareas of the United States is as high as 20%. While non-drug-resistanttuberculosis is typically 99% curable in patients with normal immuneresponses, drug-resistant tuberculosis is only about 50-60% curable. Arelated concern is drug therapy on non-drug-resistant tuberculosis forpatients who are intolerant of the drugs. In those cases, drug therapyis complicated because the drug is effective against the infection buthas serious adverse effects on the patient such as hepatitis or seriousrashes.

Another concern is raised by the increasing incidence ofnon-tuberculosis mycobacterial pulmonary infections. Many suchinfections produce symptoms similar to those of tuberculosis infections,but may be more difficult to identify and treat. Moreover, they may betransmitted through the same means as tuberculosis and tend to infectthe same types of susceptible individuals.

The transmission of the tuberculosis bacteria is accomplished almostexclusively by infected individuals expectorating microdroplets ofbacteria-containing sputum by coughing or sneezing. These microdropletsare suspended in the air and are inhaled by other individuals in thevicinity. The bacteria typically lodges in the lower lung where itproliferates, and may be disseminated to other organs as well. Themicrodroplets of sputum which contain the bacteria may be very small—onthe order of 0.01 microns. In fact, it appears that the smallestdroplets are the most effective in communicating the disease since thesmallest droplets stay airborne indefinitely and are easily inhaled tothe lower lung where they are not readily removed. Studies have shownthat aerosol droplets on the order of 1-5 microns are highly effectivevehicles for transmitting the disease.

One controversial approach to combatting the disease has been the use ofvaccines. However, the efficacy of tuberculosis vaccines is debatable.Even the trials which seemed to show some efficacy have shown lessefficacy among adults than among infants and children. An additionalobjection to widespread vaccinations is that by inducing tuberculinreactivity in the population they would confound the detection andmeasurement of infections through the use of skin tests, since skintests in vaccinated individuals would presumably result in afalse-positive. This would severely curtail the practice of preventivedrug therapy among infected patients who have not yet developed outwardsymptoms.

The airborne aspect of the disease has led toward systems for preventingthe transmission of the disease which focus on filtration andsterilizing devices. One approach is the use of masks. Simple surgicalmasks are thought to be insufficient in view of the very small size ofthe sputum microdroplets which are effective in communicating thebacteria. Instead, disposable particulate respirators are recommended.The use of masks is fraught with practical difficulties; they arephysically uncomfortable, they impair breathing (which is alreadyimpaired for many patients), and they disrupt speaking. To be effectiveat all, it would probably be necessary for the masks to be worn not justby the patients, but also by noninfected individuals. In view of thelong distances that airborne microdroplets containing viable bacteriacan travel, it would be necessary for the masks to be worn bynoninfected individuals throughout the general vicinity of a patient andnot just those in the immediate presence of a patient. Moreover, it isnot known for certain whether the use of masks would actually beeffective even if the practical problems were tolerated or overcome.

Another preventive measure which relies on the airborne aspect of thebacteria is the use of modified ventilation systems. It is currentlyrecommended that facilities used for tuberculosis patients undergocertain minimum air exchange rates, under the theory that dilution ofinfectious air with clean air will reduce the concentration of bacteriaand hence the likelihood of transmission of the disease. While thisapproach is theoretically sound, it is problematic in implementation.Modern buildings are normally designed with fixed ventilation systemswhich are not easily modified to produce the requisite air exchangerate. Even if they are suitably modified, they may be renderedineffective by an open door or by shifting air-flow patterns. A high airexchange rate also increases cooling and heating costs. Finally, thereis the issue of the ultimate disposition of the contaminated air that isremoved, and whether it is appropriate to simply release it outside thefacility.

Another approach to reducing the transmission of the disease is the useof high-efficiency filtration systems. For such a system to beeffective, however, it must employ a very dense filter to trap verysmall particles. This entails a powerful fan, high energy usage, loudnoise, and meticulous installation and maintenance. There is alsoconcern that the filters and the rest of the air-flow path maythemselves become sites of bacteria colonization.

Yet another approach to reducing the transmission of the tuberculosisbacterial employs ultraviolet light as a germicide. It was discoveredsome time ago that airborne bacteria are susceptible to ultravioletlight in wavelengths of about 254 nm. Wells S. F., On Air-BorneInfection: II-Droplets and Droplet Nuclei, Am. J. Hyg. 1934 20: 611-8;Wells W. F., Fair G. M., Viability of E. Coli Exposed to UltravioletRadiation in Air, Science 1935; 82:280-1. That finding led to thedevelopment of systems using ultraviolet light as a germicide againstairborne bacteria such as measles and tuberculosis. However, interest insuch systems diminished when later investigators were unable to obtainthe desired efficacy. Also contributing to the diminished interest insuch systems was the recognition that ultraviolet lights producedharmful ozone and also produced skin and eye irritation. With thedevelopment of streptomycin and chemotherapy for tuberculosis treatment,the belief became prevalent that tuberculosis would be eradicated andthat preventive systems would be unnecessary.

The systems that were developed using ultraviolet light as a germicideagainst tuberculosis were imprecise, marginally effective, and perhapsdangerous. The most common system simply employed ultraviolet lightsmounted on or suspended from a wall or ceiling of a room. For example, asystem employing lights suspended from the ceiling is described in somedetail in Riley, R. Z., Knight, M. and Middlebrook, G., UltravioletSusceptibility of BCG and Virulor Tubercle Bacilli, Am. Rev. of Resp.Dis., 1976, 113:413. The problems in such a system are numerous. Itrelies completely on normal air circulation in the room where it isinstalled to bring the bacteria within range of the ultraviolet light.The normal circulation in a room may be too low for the ultravioletlight to destroy a necessary proportion of bacteria, or the normalcirculation may be high enough but of a pattern that does not bring theairflow past the ultraviolet light. Moreover, there is no single test todetermine whether the circulation rate and patterns are adequate or notfor a given installation. Further, such systems quickly becomecontaminated by dust on the light bulbs which diminishes theireffectiveness. From a safety standpoint, one of the greatest concerns isthat the simple light shields used with such systems allow light to bereflected off the walls and ceiling and onto the skin and eyes of theoccupants. The degree of danger associated with the indirect ultravioletirradiation is disputed, but there is undoubtedly at least some dangerif the period of exposure is prolonged. In explaining the necessarysafety precautions, Riley, R. L. and Nordell, E. A., Clearing the Air,The Theory and Application of Ultraviolet Air Disinfection, Am. Rev.Respir. Dis. 1989 139:1286, stated:

Does germicide UV cause inflammation of skin and eyes? It can, but thestandard set by the National Institute of Occupational Safety and Health(NIOSH) is very conservative. Overhead installations must be inspectedfor ‘hot spots’ (greater than 0.2 uW/cm²) with a sensitive UV meter.Installers should anticipate readjusting fixture height up or down basedon meter readings. Baffles designed to prevent direct eye contact willalso need adjustment after the initial installation. Excessivelyreflective surfaces about fixtures may contribute to excess radiation,but this can be reduced with nonreflective paint or by spraying thesurface with stove black. If the intensity of UV does not exceed 0.2uW/cm², the likelihood of skin or eye irritation is minimal during an8-h exposure. Persons with especially sensitive skin, with systemiclupus erythematous, for example, may need to avoid exposure or takemeasures to protect their skin.

This illustrates some of the difficulties and dangers of employingultraviolet lights behind a simple light shield; the light may generatedangerous and unpredictable “hot spots”, it is not appropriate for thosewith sensitive skin or eyes, and it requires careful consideration ofthe placement and the orientation and reflectivity of the surroundingsurfaces. Finally, even if all those precautions are observed, the quoteonly indicates that skin and eye irritation is “minimal” rather thannonexistent and only for exposure periods of 8 hours. Of course, for thesystem to be effective against transmission of airborne disease in, forexample, a patient room, it would have to operate continuously and notjust for 8 hour periods. The article goes on to acknowledge that:

UV or disinfection that is inappropriately applied, poorly planned, orcarelessly used may be ineffective, dangerous, and falsely reassuring.The guidelines and precautions listed above are not intended to enable awould-be user of UV to plan, purchase, install, or check the adequacy ofa UV installation. Detailed instructions for UV installers have beenpublished. However, there is currently little commercial interest in UVfor air disinfection and, therefore, little expert guidance forcomprehensive planning and installation. Renewed consumer interest maystimulate the UV industry to correct this deficiency.

Notwithstanding the uncertainly expressed in the Riley and Nordellarticle regarding the dangers of ultraviolet radiation, that article isactually more cognizant of those dangers than much of the otherliterature on the subject. For example, the article by Riley, Knight andMiddlebrook, supra, does not even mention the dangers to the skin andeyes of ultraviolet radiation, or any precautions that should be takento minimize those dangers.

There are number of ultraviolet germicidal systems that have beenpatented, but as in the case of the scientific literature mentionedabove, those patents teach little about the dangers of ultravioletradiation and how to effectively minimize the dangers, or how toposition and operate the devices to achieve the requisite bacterial killrate to prevent transmission of disease.

For example, U.S. Pat. No. 3,975,790 by Patterson is for an ultravioletlamp fixture used in combination with a conventional commercial vacuumcleaner, and U.S. Pat. No. 4,087,925 by Bienek is for a sterilizing handdryer, in which ultraviolet lights are positioned within the housing ofa blower that is used to dry wet hands, where the blower is of the typecommonly used in commercial restrooms. The devices of Patterson andBienek seem to include little or nothing for light baffling to preventleakage of allowable light to outside the housing, and the patents teachnothing about optimal flow rates, air-exchange rates or otherinformation for the effective use of the machines. The devices areobviously intended as general, and only partially effective, sterilizingtools rather than as comprehensive and predictably effective systems.

Another patent, U.S. Pat. No. 4,210,429 by Golstein, employs a“squirrel-cage” type blower which draws air into a housing through a airintake filter, through the blower, and through a sterilization chambercontaining ultraviolet lights. The air leaves the sterilization chamber,passes through a second filter and a charcoal filter and finally exitsthrough an outlet. The specification indicates that the purpose of thedevice is to remove “pollens, lung damaging dust, smoke, bacteria andany one of a number of other irritants and micro-organisms” and that itdoes so for “particles down to 0.3 microns in size with an efficiency of99.9%”. The device is characterized as an “air purifier” rather than asa germicidal device; the use of three distinct filters including a veryfine filter for removing extremely small particles, a charcoal filterfor removing odors and a pre-filter for removing particles, isdistinguishable in design and function from the present invention. Thisextensive filtration would require a high-capacity blower to achieve anyeffective air exchange rate. The device is not specifically designed fordestroying the tuberculosis bacteria or any other specific bacteria,although it would obviously be effective in doing so to some extent.Therefore, the patent teaches nothing about the use of the device forthat purpose or the optimal flow rates or positioning of the device forthat purpose.

U.S. Pat. No. 5,074,894 by Nelson is for a hospital room to quarantinepatients with tuberculosis or other respiratory diseases caused byairborne pathogens. Although one embodiment of the system includes anair circulation circuit with ultraviolet lights, the patent is directedprimarily toward negative pressure and filtering aspects utilizinghigh-efficiency particulate air filters.

Other patents describing the use of ultraviolet light as a germicideagainst airborne bacteria include, U.S. Pat. No. 4,448,750 by Fuesting,U.S. Pat. No. 4,896,042 by Humphreys, U.S. Pat. No. 4,990,311 by Hiraiand U.S. Pat. No. 4,047,072 by Wertz, U.S. Pat. No. 4,990,313 by Pacosz,U.S. Pat. No. 3,072,978 by Minto, U.S. Pat. No. 4,227,446 by Sore, U.S.Pat. No. 3,347,0235 by Wiley, U.S. Pat. No. 4,786,812 by Humphreys, U.S.Pat. No. 4,990,311 by Hirai, U.S. Pat. No. 4,931,654 by Horng, U.S. Pat.No. 4,806,768 by Keutenedjian, U.S. Pat. No. 4,750,917 by Fugii, U.S.Pat. No. 3,757,495 by Sievers, U.S. Pat. No. 3,750,370 by Brauss, U.S.Pat. No. 3,745,750 by Arff, U.S. Pat. No. 3,744,216 by Halloran, U.S.Pat. No. 3,674,421 by Decupper, U.S. Pat. No. 3,576,593 by Cicirello,and U.S. Pat. No. 5,185,015 by Searle. Patents directed toward the useof ultraviolet light as a germicide against bacteria in water or otherliquids include U.S. Pat. No. 4,400,270 by Hillman, U.S. Pat. No.4,482,809 by Maarschalkerweerk, U.S. Pat. No. 5,102,450 by Stanley andU.S. Pat. No. 5,124,131 by Wekhof.

SUMMARY OF THE INVENTION

The present invention is an apparatus and process for destroyingairborne pathogenic bacteria such as the tuberculosis bacteria.Ultraviolet lights of a sufficient intensity are positioned within asterilization chamber where they irradiate an air stream containing thebacteria, typically in the form of suspended microdroplets of sputum.The sterilization chamber has an exit and an entrance, and a blower ispositioned preferably at the exit to draw air into the entrance andthrough the sterilization chamber and out the exit. The air circulatesbehind an intake baffle and into the sterilization chamber having a setof ultraviolet lights. An outlet baffle at the opposite side of thesterilization chamber bounces the air that passes the ultraviolet lightsback over the ultraviolet lights a second time, and around the outletbaffle to the fan. The fan then expels the sterilized air back into theroom. The air passing through the sterilization chamber is virtuallycompletely sterilized of viable tuberculosis bacteria by the chosendosimetry of the system, which is achieved by appropriately sizing thesterilization chamber employing ultraviolet lights of the correctintensity, and utilizing the right air flow rate through the blower. Theapparatus is configured to fit behind a wall in a room, or preferably,above a suspended ceiling. Air is drawn by a fan from the room into anintake duct and into the apparatus.

The sterilization chamber includes a filter on the intake side to filterout large particles such as dust, in order to minimize the contaminationof the ultraviolet light bulbs. The filter is deliberately designed notto intercept small particles such as microdroplets, since the filtercould then become a bacteria colony. The use of a low density filteralso minimizes the resistance to air flow, thereby allowing the use of asmaller, more efficient and quieter blower. With the exception of thisintake filter for removing large particulates, the apparatus preferablydoes not include any devices that would intercept and retainmicrodroplets or other small particles in a way that resists the airflow and poses the possibility of becoming a bacteria colonization site;the small particulates and microdroplets with destroyed bacteria simplypass through the apparatus and are expelled back into the environment.

Both the air intake and exhaust to the sterilization chamber are baffledso that ultraviolet light must reflect off multiple surfaces beforeexiting the sterilization chamber. The interior surfaces of the bafflesmay be light-absorptive to minimize their reflectivity and furtherlessen the possibility of ultraviolet light leaking from thesterilization chamber into the environment.

The apparatus is used in a space having a volume of air that results inan air exchange rate of preferably 12-15 air exchanges per hour. At thatair exchange rate, it has been determined that a sufficient volume ofair will circulate through the apparatus and will prevent any airstagnation in the room, that a high enough percentage of tuberculosisbacteria will be destroyed before they are inhaled by persons in theroom to prevent transmission of the disease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial cutaway view of the present invention.

FIG. 2 is a side sectional view of the present invention, taken alongline 2—2 of FIG. 1, installed in a suspended ceiling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A pictorial view of a preferred embodiment of the invention is shown inFIG. 1. The principal elements of the invention 10 include an exteriorhousing 40 having an air intake duct 42 and an air discharge duct 44, asquirrel-cage type blower 120 and set of ultraviolet lights 150 in asterilization chamber 180 within the housing 40. The air intake duct 42is preferably positioned at one end 46 of the housing and the airdischarge duct 44 is positioned at the opposite end 47 of the housing40.

As better shown in the sectional view of FIG. 2, the air intake duct 42has positioned within it a filter 60 which substantially fills theintake duct 42 so that all air drawn through the air intake duct 42 mustpass through the filter 60. The filter 60 is preferably not ahigh-density filter, but is instead designed to intercept and retainonly fairly large particulates such as dust. The purpose of the filter60 is not to allow the apparatus 10 to purify the air, but is merely tointercept dust over 10 microns in size that would otherwise contaminatethe ultraviolet light bulbs 150. In a preferred embodiment, the filteris model no. DP1-40, available from Airguard Industries located inLouisville, Kentucky. The filter 60 is retained in the air intake duct42 by means of clips, brackets or any other suitable retention means(not shown) that allow easy removal and replacement of the filter 60.

It is notable that in the preferred embodiment, there is no filter atall in the air discharge duct 44 or elsewhere downstream from thesterilization chamber. Therefore, the only filter in the preferredembodiment is the large particulate filter 60 positioned in the airintake duct 42. The apparatus 10 is designed to allow smallparticulates, including microdroplets of sputum containing bacteria thatare destroyed by the ultraviolet lights as described below, to beexpelled back into the environment. As a result, the apparatus does nothave a site that traps and allows the colonization of bacteria, whichwould require frequent cleaning or sterilization. In addition, there isvery little resistance to air flow, thereby allowing the use of arelatively small, low-energy and quiet motor and blower system, asfurther described below.

In this respect, the present system is fundamentally different fromprior art devices that are designed to remove dirt, pollen and otherparticulates and odor from the air. Those prior art systems employ denseand multiple filters and noisy high-energy blowers to indiscriminatelyremove impurities from the air. But they are not specifically for thepurpose of destroying pathogenic pulmonary bacteria such as tuberculosisand their efficiency in doing so is undocumented and questionable. Incontrast, the present system is specifically designed for destroyingbacteria such as the tuberculosis bacteria, and is highly effective inaccomplishing that using a relatively small, energy efficient, quietapparatus, but the present system makes no attempt at all to removeimpurities from the air. Even the bacteria itself is released back tothe environment once it is killed by the apparatus.

The air discharge duct 44 is preferably positioned remotely from the airintake duct 42, so that the exhausted air circulates into theenvironment rather than being immediately drawn back into the apparatus10. In the embodiment shown in FIGS. 1 and 2, the positioning of theducts 42 and 44 on opposite ends of the housing produces a circulatoryeffect through the environment of the apparatus 10 by drawing air intothe apparatus 10 through the air intake duct 42 and expelling air fromthe apparatus 10 through the air discharge duct 44, roughly in thedirection of the arrows shown in FIG. 2. The air discharge duct 44 maybe covered with a grill (not shown) to prevent the introduction of handsor objects into the air discharge duct 44 and to diffuse the air streamexhausted from there. A door 183 is positioned in the bottom of thehousing 40 as shown in FIG. 2 and is attached to the housing 40 by ahinge 185 or other suitable attachment means. The door is positioned toallow ready access to the ultraviolet lights 150 and to the filter 60 toallow them to be changed or cleaned.

The sterilization chamber 180 is baffled on the upstream side by anintake baffle 182, and on the downstream side by a pair of exhaustbaffles 184 and 187, to prevent ultraviolet light from leaking from thesterilization chamber 180 out the air intake duct 42 or air dischargeduct 44 and into the environment where it could damage the skin and eyesof patients and other persons. The baffles also improve the circulationof the air over the ultraviolet bulbs in the manner described below. Theintake baffle 182 in the preferred embodiment is an S-shaped elementfabricated from sheet metal or other appropriate material that is notdegraded by ultraviolet light. The lower portion of the intake baffle182 is curved away from the air intake duct 42 to receive the incomingair, while the upper portion of the intake baffle 182 is curved towardthe sterilization chamber 180 to allow the incoming air to flow smoothlyover the top of the intake baffle 182 and into the sterilization chamber180. The intake baffle 182 may be attached to the housing 40 at thebottom of the intake baffle 182 or at the ends.

The exhaust baffles 184 and 187 form a channel therebetween for the airto leave the sterilization chamber 180, as best shown in the sectionalview of FIG. 2. Both exhaust baffles 184 and 187 are curved with theinner side of the curve away from the sterilization chamber 180. The airpasses under the lower edge of the upper exhaust baffle 184, through thechannel defined by the upper baffle 184 and 187, and over the upper edgeof the lower exhaust baffle 187.

The upper exhaust baffle 184 may be attached to the housing 40 at thetop of the upper exhaust baffle 184 or at the ends. The lower exhaustbaffle 187 may be attached to the housing 40 at the bottom of the lowerexhaust baffle 187 or at the ends.

It can be appreciated that for any ultraviolet light to escape from thesterilization chamber 180 through the air discharge duct 44, it mustreflect off the walls of the sterilization chamber 180, reflect throughthe channel defined by the upper and lower exhaust baffles 184 and 187,and then through the blower 120 and out the air discharge duct 44. Forany ultraviolet light to escape through the air intake duct 42, it mustreflect off the walls of the sterilization chamber 180, into the spacebetween the air intake duct 42 and the intake baffle 182, through theair intake filter 60 and through the air intake duct 42. The possibilityof light escaping can be further reduced by applying an absorptivecoating or paint to the interior surfaces of the baffles 182, 184 and187 and the other interior surfaces of the housing 40.

Although the baffling described above to prevent ultraviolet light fromescaping presents a circuitous route for the passage of air from the airintake duct 42 through the sterilization chamber 180 and out the airdischarge duct 44, the baffles are still designed to minimize theresistance to air flow. Thus, as shown by the arrows in FIG. 2, the aircan flow reasonably smoothly with limited turbulence loses, therebyallowing a small, quiet and efficient blower system.

An important aspect of the embodiment shown in FIGS. 1 and 2 is that thebaffles 182 and 184 and sterilization chamber 180 are configured suchthat the air passes the ultraviolet lights twice. As shown by the arrowsof FIG. 2, the air passes the ultraviolet lights a first timeimmediately after it passes over the top of the air intake baffle 182and into the sterilization chamber. The air pathway is blocked on theopposite side of the sterilization chamber by the air exhaust baffle184. The inclined and curved surface of the air exhaust baffle, togetherwith the top wall of the housing 40, define a space 186 to receive theair after it passes the ultraviolet light a first time. The air thenreflects off the air exhaust baffle 184 and out of the space 186 andback toward the ultraviolet lights for a second pass. The air is thendrawn out of the sterilization chamber 180 by passing under the exhaustbaffle 184 and into the blower 120.

The blower 120 in the preferred embodiment is of the “squirrel-cage”type. The blower 120 draws air through its ends and propels the air outthe middle and into the exhaust duct 44. The exact size of the blowerand the motor for the blower depend on the desired use of the machineand the size of the environment in which it will be used, as furtherdiscussed below. The motor is preferably of the normal alternatingcurrent type and is in communication with the electrical system (notshown) of the apparatus, which also powers the ballasts for theultraviolet lights 152. The electrical system is ordinary, and thedetails of it will be apparent to those skilled in the wiring of lightsand motors, and it is not further described herein.

The apparatus 10 is preferably positioned in the suspended ceiling 191of a patient room as shown in a preferred arrangement in FIG. 2. Cutoutsin the ceiling 191 are provided for the air intake duct 42, airdischarge duct 44 and access door 183. The microdroplets from thepatient are expectorated from the patient into the surrounding air wherethey are suspended. The air currents produced by the apparatus 10 drawsair into the apparatus 10 from intake duct 42. The filter 60 traps largedust particles, but allows small particles to pass including the microdroplets of small bacteria-containing sputum. The air with the suspendedmicrodroplets passes through the sterilization chamber where thebacteria are destroyed by passing twice over the ultraviolet lights, andthe air along with the suspended microdroplets with the then-killedbacteria are expelled from the apparatus 10 back into the room throughthe air discharge duct 44. Because the air discharge duct 44 ispreferably positioned at one end 46 of the apparatus 10 while the airintake duct 42 is positioned at the other end 47 of the apparatus, theair being drawn into the air intake duct 42 and expelled from the airdischarge duct 44 produces a circulatory effect through the room whichincreases the flow of new unsterilized air into the apparatus. Thiscirculatory effect also helps prevent the air from short-circuiting thecirculation pattern by leaving the apparatus 10 through the airdischarge duct 44 and immediately re-entering the apparatus 10 throughthe air intake duct 42 without passing through the room.

It has been determined experimentally that transmission of thetuberculosis bacteria from an infected patient to an uninfected personcan be effectively prevented by ensuring that there are approximately 10to 15 air changes per hour in the patient room using the apparatus andpositioning described above. The phrase “10 to 15 air changes per hour”means a circulatory effect through the apparatus in which the totalvolume of air through the apparatus per hour equals the air volume ofthe room multiplied times a number between 10 and 15, inclusive. Forexample, one air change per hour in a 1,000 cubic foot room wouldrequire an apparatus through which 1,000 cubic feet of air pass perhour. Therefore, in a patient room having dimensions of 10 by 10 by 10feet for a total volume of 1,000 cubic feet, or other dimensions for atotal volume of 1,000 cubic feet, the apparatus should be capable ofcirculating through it at the rate of 10,000 to 15,000 cubic feet of airper hour.

The exact dimensions of the apparatus to achieve such a flow rate in apreferred embodiment include a housing 40 having a length of about 48inches, a height of about 15.5 inches, and a depth of about 36 inches.The air intake duct 42 is roughly 6 inches by 24 inches and the airdischarge duct 44 is roughly 6 inches by 18 inches. The opening betweenthe top of the air intake baffle 182 and the housing 40 is about 4inches, and the opening between the bottom of the air exhaust baffle 184and the housing 40 is about 4 inches. The motor is a 115 volt, 1,725 rpmmotor, and the blower 120 includes 4 by 9 inch blower wheels. Theultraviolet lights 152 are model D-36-3 by American U.V. Co.

What is claimed is:
 1. A method for destroying airborne tuberculosisbacteria in air in a room having a set of walls and a ceiling panel,comprising mounting a device behind a wall or ceiling panel of the room,filtering the air using a filter mounted on the device, drawing the airthrough a sterilization chamber in the device having at least oneultraviolet light bulb for irradiating the air with germicidalultraviolet light such that the air passes the light bulb twice, andreleasing the air including destroyed bacteria back into the room. 2.The method of claim 1, wherein the filter traps substantially noparticulates and droplets smaller than 10 microns in diameter.
 3. Themethod of claim 1, wherein the device includes an air intake opening forair to enter the device and wherein the filter is positioned between theair intake opening and the sterilization chamber whereby substantiallyall air that enters the sterilization chamber passes first through thefilter.
 4. The method of claim 3, wherein the sterilization chamberincludes an air intake baffle to prevent ultraviolet light from escapingfrom the sterilization chamber through the air intake opening and intothe room.
 5. The method of claim 4, wherein the releasing of the airincluding the destroyed bacteria back into the room is through an airexhaust opening in the device, and wherein the sterilization chamberincludes an air exhaust baffle to prevent ultraviolet light fromescaping from the sterilization chamber through the air exhaust openingand into the room.
 6. The method of claim 5, wherein at least one of theintake channel and exhaust channel is coated with an ultravioletlight-absorptive coating to absorb ultraviolet light incident thereon.7. The method of claim 1, wherein the sterilization chamber has anupstream side where the air enters the sterilization chamber and adownstream side opposite the upstream side, the upstream side anddownstream side being configured such that air enters the sterilizationchamber from the upstream side and passes the ultraviolet light bulb afirst time, reflects off the downstream side and passes the ultravioletlight bulb a second time, and exits the sterilization chamber.
 8. Themethod of claim 7, wherein the device includes a housing having a topsurface and wherein the ultraviolet light bulbs are positioned withinthe housing below the top surface, and the downstream side includes adownstream surface extending from the housing top surface downward andtoward the ultraviolet light bulbs, wherein the downstream surface andhousing top surface define a recess to receive air flowing through thesterilization chamber and to reflect the air back toward the ultravioletlight bulbs.
 9. The method of claim 1, wherein the air is drawn throughthe device at an air flow rate that is calculated to produce at leastten air exchanges per hour in the room.